This invention relates to combining optical channels and electrical channels in an oscilloscope. More particularly, the invention relates to combining the channels in a manner which enables the vertical deflection factor to be set for each channel with a single input attenuation selector switch.
Oscilloscopes contain an input attenuation selector switch which controls an attenuator section in series with the voltage input. In order to maintain a usable signal, the attenuation factor must be increased as the magnitude of the input voltage increases. A typical progression of attenuation on an oscilloscope is 1, 2, 4, 10, 20, 40, etc.
Typical optical receivers have a photodiode, a transimpedance amplifier, and one feedback resistor which results in limited dynamic range. One manner of improving the dynamic range is to include two or more feedback resistors to change the closed loop transimpedance. The feedback resistors are usually in parallel and are selectively switched into the circuit depending on the magnitude of the input photocurrent. Integrating the output of the optical receiver into an oscilloscope, however, would result in a deflection factor which is not constant. Due to varying gain modes, the output voltage of the optical receiver could not be directly used by the vertical amplifier of an oscilloscope. For example, if the optical receiver had gain modes of 3K ohms and 30K ohms, the vertical deflection could be 5 mV per division and 50 mV per division, respectively, depending upon the magnitude of the input photosignal.
What is desired is a method and apparatus for combining an optical channel and an electrical channel such that the voltage output of each channel is passed through a common vertical amplifier section and the vertical deflection factor is set by a single input attenuation selector switch.